In general, such an electromagnetic force motor of this type has been used, for instance, to drive the spool type valve for adjusting the flow or pressure of the fluid to be introduced into and discharged out of the control valve. The spool type valve and the electromagnetic force motor collectively constitute a direct operated solenoid servo valve. The direct operated electromagnetic valve is used, for example, for control of a hydraulic cylinder designed to control surfaces of an aircraft or for control of supplying a brake oil of a car.
Referring to FIG. 16, there is illustrated a typical conventional electromagnetic force motor 700 comprising a magnetic housing 710 made of a magnetic substance and having an axis 711. The electromagnetic force motor 700 further comprises a stationary magnetic member 720 made of a magnetic substance, and a movable magnetic member 740 also made of a magnetic substance and positioned in the magnetic housing 710 to be movable with respect to the magnetic housing 710 along the axis 711 of the magnetic housing 710. The stationary magnetic member 720 and the movable magnetic member 740 are partly in face-to-face relationship with and spaced apart from each other with an annular gap 701. The magnetic housing 710, the movable magnetic member 740, and the stationary magnetic member 720 collectively form a magnetic circuit unit 750 that is to allow a magnetic flux to pass therethrough. The electromagnetic force motor 700 further comprises a permanent magnet 780 located radially outwardly of the movable magnetic member 740 in the magnetic housing 710 to generate such a magnetic flux. The magnetic flux generated by the permanent magnet 780 produces a magnetic flux flow to circulate through the permanent magnet 780, the movable magnetic member 740, the stationary magnetic member 720, and the magnetic housing 710. The electromagnetic force motor 700 further comprises an electromagnetic coil 790 positioned between the stationary magnetic member 720 and the magnetic housing 710 to generate a magnetic flux with an electric current imparted thereto.
The strength of the magnetic attraction between the movable magnetic member 740 and the stationary magnetic member 720 increases in response to the decreased width of the annular gap 701, i.e. the increased moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 as shown by the curved line "U" in FIG. 17. While the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 is within the range indicated by the legend "U1" in FIG. 17, the strength of the magnetic attraction between the movable magnetic member 740 and the stationary magnetic member 720 substantially linearly increases in response to the increased moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720. While, on the other hand, the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 is within the range indicated by the legend "U2" in FIG. 17, the strength of the magnetic attraction between the movable magnetic member 740 and the stationary magnetic member 720 nonlinearly increases in response to the increased moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720. For this reason, the width of the annular gap 701 has so far been determined to ensure that the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 is maintained within the range shown by the legend "U1" in FIG. 17 so that the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 can precisely be controlled in response to the electric current imparted to the electromagnetic coil 790.
In the case that the width of the annular gap 701 is determined to ensure that the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 is maintained within the range shown by the legend "U1" in FIG. 17, the width of the annular gap 701 is larger than the width of the annular gap 701 determined to ensure that the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 is maintained within the range shown by the legend "U2" in FIG. 17 at least at a moment. This results in the fact that the strength of the magnetic attraction between the movable magnetic member 740 and the stationary magnetic member 720 becomes smaller than the desired strength, in the case that the width of the annular gap 701 is determined to ensure that the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 is maintained within the range shown by the legend "U1" in FIG. 17.
Therefore, it is necessary to increase the level of the electric current imparted to the electromagnetic coil 790 to ensure that the strength of the magnetic attraction between the movable magnetic member 740 and the stationary magnetic member 720 becomes the desired strength.
On the other hand, the density of the magnetic flux between the movable magnetic member 740 and the stationary magnetic member 720 against the electric current imparted to the electromagnetic coil 790 is shown by the curved line "V" in FIG. 18. As will be seen from FIG. 18, the magnetic circuit unit 750 is saturated with the magnetic flux while the level of the electric current imparted to the electromagnetic coil 790 is within the range shown by the legend "V2". This means that the density of the magnetic flux between the movable magnetic member 740 and the stationary magnetic member 720 non-linearly increases in response to the increased level of the electric current imparted to the electromagnetic coil 790 within the range shown by the legend "V2" in FIG. 18. Therefore, the cross-sectional area of the magnetic circuit unit 750 has so far been determined to ensure that the density of the magnetic flux between the movable magnetic member 740 and the stationary magnetic member 720 substantially linearly increases in response to the increased level of the electric current imparted to the electromagnetic coil 790 under the state that the level of the electric current is maintained within the range shown by the legend "VI" in FIG. 18 so that the moving distance of the movable magnetic member 740 with respect to the stationary magnetic member 720 can precisely be controlled in response to the electric current imparted to the electromagnetic coil 790.
On the other hand, the range shown by the legend "V1" in FIG. 18 increases in response to the increased cross-sectional area of the magnetic circuit unit 750.
Therefore, it has also been necessary to increase the cross-sectional area of the magnetic circuit unit 750 to ensure that the strength of the magnetic attraction between the movable magnetic member 740 and the stationary magnetic member 720 becomes the desired strength.
The fact that at least one of the width of the annular gap 701 and the cross-sectional area of the magnetic circuit unit 750 are relatively large results in the fact that the size and weight of the electromagnetic force motor 700 become relatively large.
In the meantime, the direct operated solenoid servo valve is desired to become as small as possible resulting from the fact that the direct operated solenoid servo valve is required to be as light as possible particularly when it is used as a direct operated solenoid servo valve in the aircraft.
It is, therefore, an object of the present invention to provide an electromagnetic force motor, which is reduced in size while effectively maintaining its performance at almost the same level as that of the conventional electromagnetic force motor.
It is another object of the present invention to provide a method of manufacturing an electromagnetic force motor which can effectively work at almost the same level as that of the conventional electromagnetic force motor with its size reduced.